AuthorTopic: Summing (Read 19800 times)

It is best to have that final truncation done with dither. At first glance, that 1/2 least significant bit may not "look as bad". But it is worse then it looks, and here is why:

If you add dither, the overall noise energy will be a little higher, but it will be "spread out" evenly across the spectrum.

I agree any final truncation should be done with dither for the best sound. The interesting word though is "final". I suspect 'final' applies to any mix within the DAW for example just after a bounce. If it does then it implies noise will build up with successive digital bounces just the way it used to in analog days.

Ian

Hi Ian,

In principle, any truncation, should be dithered, because as I pointed out, not truncating may lead to energy concentration in some specific frequencies. The energy concentration I am talking about may have peaks far above the noise floor of a dithered signal. Again, the average value and the rms value of the un dithered signal may be lower, but the peaks of the un dithered signal may be higher then the dithered noise floor.

So in principle, any word length reduction (truncation)should be dithered.

There are times when one decides not to dither, such as when the noise content is already dominating the lower bits of the truncated signal. But from a pure standpoint, it goes beyond just the final truncation.

However, when it comes to the final truncation, the use of dithered noise shaping should be reserved for the final truncation. Noise shaping dither is far better then dithers without noise shaping, because the dither noise is being pushed away from hearing sensitive ear regions (such as 1-3KHz) to places we do not hear as well (such as say 16-22KHz). But one can not noise shape again and again, you can get the desired shaping by doing it only once, in the final truncation.

Dan from what you are saying, I am now picturing the notion of all truncation should have dither as something akin to analog. Because somebody mentioned before the dither adds up, and you mentioned that in essence that is better than the peaks and prevalence of odd harmonics, maybe that dither adding up emulates the sort of character analog gives to audio. I dont mean exactly but along the idea of sometimes imperfections of a system can sound nice to cover really ugly imperfections of audio. So is the case with dithering all truncations. Am I off here?

I should ask, when you say truncation does that include going from the inherent floating bit of a DAW down to the file bit depth, say 64 or 32 back down to 24? should a simple dither be added here?

I should ask, when you say truncation does that include going from the inherent floating bit of a DAW down to the file bit depth, say 64 or 32 back down to 24? should a simple dither be added here?

In purist terms, yes. In the real world, it will probably make no difference (depending on the signal and/or what you do with it after the conversion).

If you're going to send the result to a DAC and out through your playback system, then you'll be adding more than 1 bits worth of analogue noise anyway, so that will completely swamp any artifacts from truncation.

If you were converting to 16 bits it would be a different story. It would also be different if you took that 24 bit integer stream applied gain to it (effectively the same thing).

I should ask, when you say truncation does that include going from the inherent floating bit of a DAW down to the file bit depth, say 64 or 32 back down to 24? should a simple dither be added here?

In purist terms, yes. In the real world, it will probably make no difference (depending on the signal and/or what you do with it after the conversion).

If you're going to send the result to a DAC and out through your playback system, then you'll be adding more than 1 bits worth of analogue noise anyway, so that will completely swamp any artifacts from truncation.

If you were converting to 16 bits it would be a different story. It would also be different if you took that 24 bit integer stream applied gain to it (effectively the same thing).

Jon and I are in agreement about "purist" vs, "practical", and of course it brings a question: where does one draw a "line" between the two.

Let's examine adding random noise into a signal that already contains random noise. Clearly, if the already existing random noise is higher in amplitude then the dither noise, the dither will be "swamped", "overpowered", "buried" by the already present noise in the signal. And if the signal itself has very little noise, then the dither can "rule", "overpower" the noise in the signal.

That is why Jon is suggesting that truncating to 24 bits and to 16 bits are different stories. As a rule, the signal out of a converter will have some of the least significant bits contain noise. You can take such signals and run them through all sorts of processing in a DAW, with very wide mix bus (it can be 1000 bits wide), but if bit 20-24 recorded noise, the noise is still there at bit 20-24, no matter what you do. So setting the dither at say bit 24 may make little sense. But assuming that bit 16 was noise free, it makes sense to dither at bit 16 when you truncate to 16 bits. The need for dither exist only at the point that a signal is noise free.

So that brings us back to: how many bits is noise free? That depends a lot on the micpre input noise floor, on the gain setting of the micpre (see an article on my company web under tutorials), and it may depends on the AD. In practice, a recorded sound will have at lease the last 4 bits carry noise and no sound at all. There is still a long way from 20 bits "true", to 16 bits format, so dither does have it's place...

Here is a plot showing what I said earlier. It is an FFT showing a 3 LSB peak to peak 1KHz sine wave tone. Both the red an blue plots share the same tone at 1KHz (the blue 1KHz tone "covers" the red 1KHz tone almost completely so it is not easy to see, so take my word for it).

But other then the energy at 1KHz, the RED (Truncated to 16 bits with NO DITHER), and the BLUE (Dithered and truncated to 16 bits) are very different plots.

The RED (no dither) has less total noise, and the overall noise floor is lower then the BLUE (dithered).

But the RED (no dither) has some peaks that go way above the Blue noise floor. In this example, there is a "big peak" around 9KHz, way above the dithered noise floor.

Note that those peaks keep changing with the music in a very non musical manner. Also, such peaks "disappear" when you raise the signal amplitude to a high enough level. So dither is not going to help loud music, it helps signals such as "the tail end" of a decaying piano note....

The point is: While dither adds to the overall noise, the truncation without dither result with some undesirable energy that peaks way above the dithered signal noise. The dithered noise is "spread out" sort of evenly accross the audio. You hear it as noise. The "energy peaks" of the un dithered signal "stand out". They don'y belong there and they have a "non random sound" which is undesirable.

Right, non random by its nature will even stick out audibly speaking, yes? So the dither sort of act as stilts while keeping a "constant" random flow under the actual music; keeping it steady in some way. I've listen to a file with and without dither and with the dither, certain harsh points were gone? or, not as harsh? point is, it sounded more natural with the dither. Thats why it makes me think of analog and possibly the kind of "stilt" and electronic noise personality analog circtuiry possesses. From what I learned here, i will assume any one track within a mix i process outside the box I will not look to add noise or dither to ITB because of the conversion adding its own dither.

Right, non random by its nature will even stick out audibly speaking, yes? So the dither sort of act as stilts while keeping a "constant" random flow under the actual music; keeping it steady in some way. I've listen to a file with and without dither and with the dither, certain harsh points were gone? or, not as harsh? point is, it sounded more natural with the dither. Thats why it makes me think of analog and possibly the kind of "stilt" and electronic noise personality analog circtuiry possesses. From what I learned here, i will assume any one track within a mix i process outside the box I will not look to add noise or dither to ITB because of the conversion adding its own dither.

Be careful not to jump into the wrong conclusion.

You first need to know how much (or little) noise you already have in the signal. As rule, I would not worry too much about dither when truncating to 24 bits. I would certainly dither when going to 16 bits. There are a lot of "cases in between".Usually, when in doubt, add dither. It does not raise the noise level that much, and the possible benefits tend to far outweigh the little increase in noise.

Also, adding dither again and again is not as bad as it sounds initially. Say you added dither and it cost you a little by increasing the noise floor by XdB. The next time you add dither (for the same word length) the noise will not go up by another Xdb. The second time it will go up by a lot less then Xdb. It does not add linearly. Losing another XdB (for a total of 2X dB) will take adding dither 4 times. Losing 3XdB (where XdB is dithering once) will require 8 times of adding dither...

So once you added dither, much of the increase in noise floor is already done... It is a bit complected, I wish I could simplify it, sorry....

Analog noise and what it does is a whole subject to itself, maybe another for thread another time..

yes I understand. i think i was making digital analogous to analog but without the coloration and circuitry noise and harmonics of analog. if that makes sense.. i think it makes sense then, that without previous noise build up or dither 20x, some dither and some times, while remaing 24 bit or higher can be helpful.

In principle, any truncation, should be dithered, because as I pointed out, not truncating may lead to energy concentration in some specific frequencies. The energy concentration I am talking about may have peaks far above the noise floor of a dithered signal. Again, the average value and the rms value of the un dithered signal may be lower, but the peaks of the un dithered signal may be higher then the dithered noise floor.

So in principle, any word length reduction (truncation)should be dithered.

Hi Dan,

I agree. I wonder what happens though in a typical DAW? Suppose you make a stereo sub mix of a drum kit, for example; presumably this sub mix has been dithered during its creation? Repeat this for other elements of the music then make a final stereo mix of the sub mixes. Presumably dither is applied again? I know the answer will be DAW specific but there must be some general principles to follow.

In principle, any truncation, should be dithered, because as I pointed out, not truncating may lead to energy concentration in some specific frequencies. The energy concentration I am talking about may have peaks far above the noise floor of a dithered signal. Again, the average value and the rms value of the un dithered signal may be lower, but the peaks of the un dithered signal may be higher then the dithered noise floor.

So in principle, any word length reduction (truncation)should be dithered.

Hi Dan,

I agree. I wonder what happens though in a typical DAW? Suppose you make a stereo sub mix of a drum kit, for example; presumably this sub mix has been dithered during its creation? Repeat this for other elements of the music then make a final stereo mix of the sub mixes. Presumably dither is applied again? I know the answer will be DAW specific but there must be some general principles to follow.

Ian

I am not sure what all the DAW do. Ideally, the DAW would keep everything without truncation until the end of the process, when one MUST truncate to "fit" the data into some format (such as AES or SPDIF).

I do not know what word length is used for the "sub mix". Ideally one can keep it stored with wide words, such as offered by the mix bus. Dither is needed only when you reduce the word length (less bits).

Say you have some sub mix of some word length, and you want to add to it another track, or another sub mix, or do some EQ... The DAW should allow you to do such operations, and when the operations call for more bits (to the left and or to the right), so at this point there is no reason to truncate, thus no dither is required.

From dither standpoint, I would treat a sub mix the same way I treat a single track. It has some given number of bits. The more significant bits (hopefully 15-21 bits) carry music, the lower bits (22 to whatever) carry noise.

I have said it before a number of times, but here again:There is the issue of bits for digital processing and DAW.There is the issue of bits for conversion.

The DAW needs a lot of bits, to allow adding tracks, amplifying / attenuating / EQ / reverb.... all take "work space", so you need a lot of bit to do the processing. Once done, you reduce the number of bits.

The conversion bits is a different story. Say your music is recorded with a 24 bit format where the noise floor is such that only the top 16 bits carry music. In this example, the bottom 8 bits are of no value. All that noise is due to limitations such as mic pre noise, AD noise or what not...

Just because one loads such a file into a DAW with say 48 bits bus, does not mean at all that the extra DAW bits will make the music on that track end up with better then 16 bit. It will not. Say you decide to do an EQ and boost the 10-20KHz by 6dB. The computation will immediately call for a lot more bits, one on the MSB side, and many on the LSB. However, when you boost the music signal over 10-20KHz, you also boost the noise over 10-20KHz...

So how many bits? For conversion, 20 bits result out of an AD is very rare! For DAW, 20 bits is very limiting, you need much more then that....

I am not sure what all the DAW do. Ideally, the DAW would keep everything without truncation until the end of the process, when one MUST truncate to "fit" the data into some format (such as AES or SPDIF).

snip

I have said it before a number of times, but here again:There is the issue of bits for digital processing and DAW.There is the issue of bits for conversion.

Of course, it makes sense for the DAW to use as many bits as necessary until it has to dither. I suppose on PC DAWs with unlimited tracks and FX this is mostly not an issue because dither probably therefore only needs to be applied to the final mix. Everything else is just processing of the original data.

I suppose my interest in intermediate dithering is born out of the way I tend to work plus the fact that I have done a lot of bouncing in 40 years of recording. I don't use a PC, I use a stand alone DAW, an AKAI DPS24. For various reasons I tend to bounce and print several submixes. These, along with the remaining tracks are mixed to the final stereo mix. Clearly it is important to the quality of my final mix that the printed bounced sub mixes are dithered, and not truncated, before they are printed.

i know this is a dumb question, but i still wonder where floating point falls? doesnt floating point imply the virtual bit depth at which the daw processes and not the actual file bit depth? so one could have a 24 bit file bit depth but processes at 32 bit float. in this case, whats happening between the 32 bit float processing point and after the rendering down to the 24 bit depthe actual file? does a simple dither (not shaped dither which should be for the final product)apply here? is that even considered truncation?

I am not sure what all the DAW do. Ideally, the DAW would keep everything without truncation until the end of the process, when one MUST truncate to "fit" the data into some format (such as AES or SPDIF).

snip

I have said it before a number of times, but here again:There is the issue of bits for digital processing and DAW.There is the issue of bits for conversion.

Of course, it makes sense for the DAW to use as many bits as necessary until it has to dither. I suppose on PC DAWs with unlimited tracks and FX this is mostly not an issue because dither probably therefore only needs to be applied to the final mix. Everything else is just processing of the original data.

I suppose my interest in intermediate dithering is born out of the way I tend to work plus the fact that I have done a lot of bouncing in 40 years of recording. I don't use a PC, I use a stand alone DAW, an AKAI DPS24. For various reasons I tend to bounce and print several submixes. These, along with the remaining tracks are mixed to the final stereo mix. Clearly it is important to the quality of my final mix that the printed bounced sub mixes are dithered, and not truncated, before they are printed.

Ian

I have not examined in detail the internal working of a DAW, but at first glance, I would think that more bits calls for much more then then just more storage and a wider buss. It may call for a more powerful compute engine.

I am not sure what all the DAW do. Ideally, the DAW would keep everything without truncation until the end of the process, when one MUST truncate to "fit" the data into some format (such as AES or SPDIF).

snip

I have said it before a number of times, but here again:There is the issue of bits for digital processing and DAW.There is the issue of bits for conversion.

Of course, it makes sense for the DAW to use as many bits as necessary until it has to dither. I suppose on PC DAWs with unlimited tracks and FX this is mostly not an issue because dither probably therefore only needs to be applied to the final mix. Everything else is just processing of the original data.

I suppose my interest in intermediate dithering is born out of the way I tend to work plus the fact that I have done a lot of bouncing in 40 years of recording. I don't use a PC, I use a stand alone DAW, an AKAI DPS24. For various reasons I tend to bounce and print several submixes. These, along with the remaining tracks are mixed to the final stereo mix. Clearly it is important to the quality of my final mix that the printed bounced sub mixes are dithered, and not truncated, before they are printed.

Ian

First order, reducing wordlength is truncation, with or without dither added. You dither the truncation, it's not a one or the other scenario. IOW, dithering a 24 to 16 bit wordlength is still a truncation, just a dithered truncation. Second, I also use stand alone HD-R's and prefer to print submixes and plug-in processing, as you do when I'm in a DAW, it eases CPU taxation and eliminates the power needed to run processors in real time, I get better performance in my systems that way, it takes a little longer, but I feel it's worth it. However not all printed processes while remaining at the native rate in the DAW need user applied dither. For example if you are operating at 32 bit, process and print at 32 bit, you won't need to add dither until you truncate the wordlength, no word reduction, no dither needed in most cases that I'm aware of. Someone can correct me if they know different.

I suppose my interest in intermediate dithering is born out of the way I tend to work plus the fact that I have done a lot of bouncing in 40 years of recording. I don't use a PC, I use a stand alone DAW, an AKAI DPS24. For various reasons I tend to bounce and print several submixes. These, along with the remaining tracks are mixed to the final stereo mix. Clearly it is important to the quality of my final mix that the printed bounced sub mixes are dithered, and not truncated, before they are printed.

Ian

Quote:

I have not examined in detail the internal working of a DAW, but at first glance, I would think that more bits calls for much more then then just more storage and a wider buss. It may call for a more powerful compute engine.

The beauty of printing the process rather than running it in real time, is that the power of the computer engine need not be extravagant. The less power and processing speed, the longer it typically takes to print, but that's about it from my experience of printing processes, rather than trying to run a bunch in real time. The benefit is that the computer can take the time it needs to process the signal effectively. For real time processing, yes, the more process' that you run, the more power that you'll need, for printing processes not so important as the computer doesn't have to keep up with the power needed for real time processing. You have less chance of errors to occur and you completely eliminate the latency that you get from processing in real time chaining through various processors.